In development of pharmacological agents aiming at inhibiting the activity of dipeptidyl peptidase IV (hereinafter, may be referred to as “DPP-IV”) or pharmaceutical products (mainly drugs for diabetes) with glucagon-like peptide-1 (GLP-1) as a pharmacological agent, the measurement of the concentration of GLP-1 that is active in the blood (GLP-1 (active)) or the concentration of total GLP-1 (GLP-1 (total)) is often carried out for the purpose of measuring parameters for drug efficacy evaluation and the concentration of drugs.
Mechanisms of action of DPP-IV inhibitors involve increasing the concentration active form GLP-1, which has a property of being decomposed by DPP-IV, by inhibiting the activity of DPP-IV and promoting insulin secretion by GLP-1 to promote utilization of sugars. Thus, by measuring the concentration of GLP-1, effects of the action of the DPP-IV inhibitor can be evaluated.
As the active form GLP-1, there are known to be GLP-1 (7-36 Amide) and GLP-1 (7-37). Measurement of these is widely carried out by an immunoassay method using a specific antibody (ELISA method: a kit manufactured by Merck Millipore or the like). However, non-specific reactions often take place and it has thus been known that, in order to measure an accurate concentration in the blood plasma, a certain pretreatment is required to be carried out to get rid of the non-specific reaction (Non-patent Document 1). In general, what is considered as the non-specific reaction in the ELISA method is one ascribed to a sample such as the blood, one ascribed to an antibody and/or reagent, one ascribed to a microtiter plate, or the like. In conventional methods, the non-specific reactions are considered to be ascribed to the sample. These are removed from the sample by a pretreatment method using a column and then the ELISA method is carried out. This pretreatment method separates substances causing the non-specific reaction from GLP-1 by a solid phase column or ethanol extraction operation. Yet, an amount of blood plasma necessary for one examination is as high as 300 μL; and the method requires complicated operations and time, which operations include reagent preparation, separation operation, drying to solidify using a nitrogen gas, and re-dissolution; and also requires the skill of those who are involved in the operations. From the above, it cannot be said that the conventional method is a simple and low-cost measurement method. Further, because a loss of 20 to 30% of GLP-1 in a sample is unavoidable by this pretreatment operation, the measurement value of GLP-1 is considered to be about 70 to 80% based on a true value, which has been problematic in terms of accuracy as well.